1. Field of the Invention
The present invention relates to wavelength division multiplex transmission systems for transmitting optical signals having different wavelengths by using a wavelength division multiplex technology.
2. Description of the Background Art
In recent years, in order to cope with an increase in demand for large-volume data, a high-density wavelength division multiplex transmission system (hereinafter referred to as a DWDM system) has been rapidly widespread.
In the DWDM system, a plurality of optical signals are transmitted with their wavelengths aligned at narrow intervals of 1 nm or less. Therefore, transmission characteristics of a wavelength demultiplexer for demultiplexing and extracting the optical signals have to be steeply attenuated outside a desired wavelength band. For this reason, even if the wavelength of the optical signal is slightly deviated from the transmission center waveform of the waveform demultiplexer, excessive losses disadvantageously occur. In order to cope with this disadvantage, the wavelength of light output from a semiconductor laser should be accurately controlled. Conventionally, a wavelength control technique has been employed in the wavelength division multiplex transmission system in order to control the wavelength of an optical signal to be transmitted so that the wavelength is at an appropriate wavelength. Hereinafter, a conventional wavelength control technique is specifically described.
FIG. 22 illustrates the configuration of a conventional wavelength division multiplex transmission system. In FIG. 22, a conventional wavelength control means includes first through N-th (N is an integer of 2 or more) transmitting sections 2211 through 221N, first through N-th controllers 2221 through 222N, a wavelength multiplexer 223, an optical transmission path 224, a wavelength demultiplexer 225, and first through N-th receiving sections 2261 through 226N. Here, the first transmitting section 2211 includes a data signal source 2201, an electrical-optical converter 2202, and an optical brancher 2203. The first controller 2221 includes an optical filter 2204, an optical-electrical converter 2205, a wavelength detector 2206, and a wavelength controller 2207. Although not shown, a k-th (k is an integer from 2 to N) transmitting section 221k and a k-th controller 222k have the same structure as the first transmitting section 2211 and the first controller 2221, respectively.
The operation of the conventional wavelength control means illustrated in FIG. 22 is described below. The first through N-th transmitting sections 2211 through 221N convert a data signal to be transmitted to optical signals having different wavelengths. The first through N-th controllers 2221 through 222N are provided correspondingly to the first through N-th transmitting sections 2211 through 221N, so as to control the wavelength of the optical signal outputted from the corresponding transmitting section. The wavelength multiplexer 223 wavelength-multiplexes first through N-th optical signals supplied by the first through N-th transmitting sections 2211 through 221N. The optical transmission path 224 leads an optical signal output from the wavelength multiplexer 223. The wavelength demultiplexer 225 is provided with N output terminals, and has wavelength bands that give different maximum transmission rates for the respective output terminals. The wavelength demultiplexer 225 demultiplexes the optical signal transmitted via the optical transmission path 224 into the first through N-th optical signals for output from the output terminals. The first through N-th receiving sections 2261 through 226N are connected to the output terminals of the wavelength demultiplexer 225, and each convert the optical signal to an electrical signal (data signal).
The operation of each transmitting section and controller is described next below. In the following, descriptions are made to the operation of the first transmitting section 2211 and the first controller 2221, and these descriptions are also applicable to the operation of the other transmitting sections and controllers. In the first transmitting section 2211, the data signal source 2201 generates a data signal to be transmitted. The electrical-optical converter 2202 converts the electrical signal output from the data signal source 2201 to an optical signal. The optical brancher 2203 branches the optical signal output from the electrical-optical converter 2202 into two, one being supplied to the wavelength multiplexer 223 and the other being supplied to the first controller 2221.
In the first controller 2221, the optical filter 2204 possesses predetermined transmission characteristics for passing the optical signal supplied by the optical brancher 2203 for output to the optical-electrical converter 2205. The optical-electrical converter 2205 converts the optical signal output from the optical filter 2204 to an electrical signal for output to the wavelength detector 2206. Here, the predetermined transmission characteristics of the optical filter 2204 are such that a transmission rate is varied depending uniquely on the wavelength of the input optical signal. That is, the level of the signal output from the optical-electrical converter 2205 is varied depending on the wavelength of the optical signal supplied to the optical filter. Based on such characteristics, the wavelength detector 2206 outputs wavelength information. The wavelength controller 2207 controls the electrical-optical converter 2202 based on the wavelength information output from the wavelength detector 2206 so that the level of the electrical signal output from the optical-electrical converter 2205 has a predetermined value. With this control, the optical signal output from the first transmitting section 2211 is adjusted to have a predetermined wavelength.
As another example of conventional wavelength multiplex techniques, a wavelength control apparatus disclosed in Japanese Patent Laid-Open Publication No. H11-31859 (1999-31859) is described below. FIG. 23 illustrates the configuration of this wavelength control apparatus. The wavelength control apparatus includes a semiconductor laser 231, a cut filter 232, a beam splitter 233, an optical band-pass filter 234, photodiodes 235 and 236, an output power ratio calculator 237, and a wavelength controller 238. Light output from the semiconductor laser 231 first passes through the cut filter 232 having transmission characteristics as shown in FIG. 24A, and then enters the beam splitter 233. The beam splitter 233 passes part of the injected light and reflects the rest. The light passing through the beam splitter 233 is used for signal transmission, and the reflected light is used for wavelength monitoring, which is described below. The light reflected by the beam splitter 233 first enters the optical band-pass filter 234 having transmission characteristics shown in FIG. 24B. Light passing through the optical band-pass filter 234 enters the photodiode 235, and reflected light enters the photodiode 236. The wavelength dependency of the photodiode 235 at a light-receiving level can be given by the product of a transmission rate of the cut filter 232 and a transmission rate of the optical band-pass filter 234, which is as illustrated in FIG. 24D. On the other hand, the wavelength dependency of the photodiode 236 at a light-receiving level can be given by the product of the transmission rate of the cut filter 232 and a reflection rate of the optical band-pass filter 234, which is as illustrated in FIG. 24E. Outputs from the photodiodes are supplied to the output power ratio calculator 237. Here, an output level from the photodiode 235 is taken as A, and an output level from the photodiode 236 is taken as B. The output power ratio calculator 237 calculates an output power ratio of (A−B)/(A+B) for output as a wavelength monitor signal (refer to FIG. 24F). The wavelength controller 38 controls a wavelength of the light emitted from the semiconductor laser 231 so that the wavelength monitor signal has a predetermined value X. With the predetermined value X being set to a value corresponding to a transmission center wavelength of the wavelength demultiplexer, the wavelength λ2 of the light emitted from the semiconductor laser can be appropriately controlled.
As described above, in the conventional wavelength control technique, each transmitting section has to be provided with an optical filter for controlling the wavelength of an output optical signal, an optical brancher, and an optical-electrical converter (photodiode, for example), or a wavelength locker composed of the above-mentioned components, in order to accurately control and stabilize the wavelength of each optical signal. However, these optical devices are generally expensive. Therefore, extremely high costs are disadvantageously required for each transmitting section. Furthermore, with these expensive optical devices being required for each transmitting section, cost effectiveness of the entire wavelength division multiplex transmission system is significantly degraded as the size of the system is increased.
Still further, the wavelength demultiplexer (the wavelength demultiplexer 225 of FIG. 22) placed on the optical transmission path has a characteristic that its transmission characteristics are changed depending on the ambient temperature or the like. In the conventional wavelength division multiplex transmission system, however, such a characteristic of the wavelength demultiplexer is not considered by the wavelength controller. Therefore, the conventional system does not have any means for improving and stabilizing signal transmission characteristics having degraded by changes in the transmission characteristics of the wavelength demultiplexer. Particularly, when a wavelength locker is used, the wavelength locker is manufactured so as to be specifically targeted for a predetermined wavelength, which cannot be easily reset. For this reason, it is difficult to mitigate degradation of the signal transmission characteristics in the wavelength division multiplex transmission system.